This proposal addresses abnormal development of B cells and B cell precursors in murine models of systemic lupus erythematosus (SLE). We hypothesize that NZB and (NZB x NZW)F1 autoimmune mice undergo a progressive decline in both the number of B cell precursors detected within the bone marrow and in the mitotic activity of these precursor cells. This may result from either 1) failure of B cell precursors respond to developmental signals: 2) failure of the bone marrow stroma to adequately support B lineage proliferation and development; or 3) abnormal down-regulation of B lineage precursors via T cells and/or regulatory Ly1/Mac1 lineage B cells. In order to test this hypothesis, we will: A) Use vincristine induced metaphase arrest (stathmokinetic) assays to assess the mitotic activity of B cell precursor cells in both normal and NZ autoimmune bone marrow as a function of age. B) Define whether the mechanisms responsible for defective B lymphopoiesis in NZ mice operate at the precursor cell level or upon the non-lymphoid stroma necessary for the support of B lineage cell proliferation and differentiation. These experiments will use in vitro short term and long term bone marrow cell culture systems to allow differentiation of NZ B lineage cells with normal bone marrow stroma and vice versa. C) Determine the capacity of NZ bone marrow stromal cells to secrete cytokines, including IL-7, necessary for maintenance of pre-B cell growth. These experiments will use bioassays and also will quantitate ILl-7 mRNA levels in normal vs. NZ stroma. D) Evaluate the capacity of NZ T cells to secrete excessive quantities of lymphokines which inhibit proliferation of B lineage precursor cells. Specific bioassays for both IL-2 and IL-4 will be performed since these lymphokines may influence B lineage cell development and growth. E) Assess the capacity of NZ Ly1/Mac1 bearing B cells within the bone marrow to down-regulate the proliferation and generative capacity of bone marrow B lineage cells. These experiments will use in vivo model systems in which NZ Ly1/Mac1 B cells will be expanded in normal recipients (e.g., [NZB x BALB/c]F1 or immunodeficient SCID mice). The capacity of normal B lineage precursor cells to proliferate and develop in the presence of NZ LY1/Mac1 B cells will then be determined. With this experimental plan, we will test the above hypothesis and establish the mechanisms by which B lineage cell development in NZ mice is decreased and the possible role of this defect in establishment of lymphocyte autoreactivity.